There are many applications in which a periodic waveform swept through a range of frequencies is required. For example, to characterize the frequency response of a circuit, it may be desirable to apply a sweep signal to the circuit and monitor the response.
Analog circuits which can generate such a sweep signal of significant duration suffer the drawbacks of being unable to operate from a presettable start phase, which would enable coherent waveforms to be generated simultaneously, and of being unable to attain very high sweep rates. The term "coherent" refers to a specified phase relationship between the waveforms.
Digital arbitrary function generators can produce any desired waveforms, including swept frequency waveforms with controlled phase. Precomputation and storage of the signal is required to generate high frequency signals because the digital processors are unable to perform the necessary computations in real time. A drawback of such systems is that the entire signal must be stored in high speed digital memory, and the duration of the signal is thus limited by memory space. For example, in such a system the memory requirement needed to produce a swept signal from zero Hertz to 20 MHz over 60 seconds, using a clock frequency of 100 MHz, is 6 gigabytes of high-speed memory. Such a memory is not feasible today and would exceed the addressing capabilities of even a 32-bit computer.
There exists a need for a sweep generator which can generate in real time a linear sweep signal where the required frequencies are in the MHz range and the sweep duration is many seconds.